Anglo-Swedish pharmaceutical company AstraZeneca says it regrets the decision of the European Commission to take it to court over its delivery of COVID-19 vaccines to European Union member states, and has promised to defend itself.

A claim was lodged in the Belgian courts last week and the first hearing will be on Wednesday, taking long-running tensions between the two sides to a new level.

The Commission decided to organize EU vaccine supply and distribution centrally, but was unhappy when the company managed to supply just one-quarter of the expected 120 million doses of vaccine in the first three months of the year, because of difficulties at a production facility in Belgium.

More bad blood was caused by AstraZeneca's refusal to divert supplies that had been manufactured in the United Kingdom, which had signed its own contract with the company before the Commission acted.

"Following an unprecedented year of scientific discovery, very complex negotiations, and manufacturing challenges, our company is about to deliver almost 50 million doses to European countries by the end of April, in line with our forecast," said a statement issued by the company.

"AstraZeneca has fully complied with the advance purchase agreement with the European Commission and will strongly defend itself in court. We believe any litigation is without merit and we welcome this opportunity to resolve this dispute as soon as possible."

The Commission responded by saying the company had failed to respect some terms of the contract or been able to show a strategy for living up to its delivery schedule.

"What matters to us in this case: there's a speedy delivery of a sufficient number of doses that European citizens are entitled to and have been promised on the basis of the contract," said a spokesman.

The United States government has said it will share up to 60 million doses of its own supply of AstraZeneca vaccine with other countries as they become available. The vaccine has not yet been cleared for use by US medical authorities, but there is already a stockpile of doses that will be made available for export once it has been through a safety review.

Previously, President Joe Biden had pledged to make around four million doses available to neighbors Canada and Mexico, both of which have cleared the vaccine for use, and the latest donation comes as the pictures of India's COVID-19 crisis have been seen around the world.

Biden had what the White House called a "warm and positive" phone call with India's Prime Minister Narendra Modi on Monday, and has already begun sending medical supplies.

However, the Financial Times reports that supplies of the AstraZeneca vaccine could be delayed after what it called manufacturing problems at the Baltimore plant owned by Emergent BioSolutions, where previous issues have led to other vaccine doses being ruined.